Known wireless sensor systems place a sensor in a remote location, where it is impractical to perform an accuracy check on the deployed sensor due to the environment in which the sensor is placed. Many environments do not allow for a separate reference reading of the parameter being sensed due to the remote location's environmental or spatial constraints. In one example, the remote location is a body. Wireless sensor systems may generally include a reader unit or device that may have a configuration where it is placed in a use condition for taking readings from the sensor and in a resting condition in which it is not communicating with the sensor. For example, a reader unit may be handheld or battery operated and be adapted for use a few minutes each day. This reader unit may also be configured to sit on a recharging (“docking”) station during times of non-use. However, these reader and sensor systems may be field-deployed such that it may be impractical to conduct factory testing and calibration to determine or correct the accuracy of the system. It may also be impractical or cost prohibitive to send a service technician to the field to check system accuracy. Thus, it is desirable for the wireless sensor/reader system to be able to conduct a self-test to assess its own performance and accuracy. It is also desirable that the self-test require minimal extra equipment, and minimal effort by the user of the device. It is also desirable that the self-test does not interrupt, slow down, delay, or otherwise disturb the reader's sensor interrogation function.
Reader units may be placed in a standard location, such as a charging or docking station, when not actively communicating with the sensor. The need for simple, cost-effective, low-effort self-test in the field is shared by sensor/reader systems incorporating many types of wireless technology: active & passive sensors, continuous wave (CW) & modulated data transmission, and analog & digital type systems.
In one application, passive wireless sensor systems may employ resonant circuit technology. These systems utilize a passive wireless sensor in remote communication with excitation and reader circuitry. Often the wireless sensor is placed in a specific location, such as within the human body, to detect and report a sensed parameter. The sensed parameter varies a resonant circuit frequency of the wireless sensor. A reader device samples and analyzes the resonant frequency of the wireless sensor to determine the sensed parameter.
Passive wireless sensor systems may be pressure monitoring devices for use by themselves or incorporated into other medical devices including, without limitation, pacemakers and defibrillators. In one embodiment, a medical device includes one or more pressure sensors that is configured to be positioned at a desired location within the human body. The pressure sensor may be fabricated using a microelectromechanical systems (MEMS) technique and may be configured to transmit wireless data to an external receiver/reader to facilitate data transmission of parameter measurements to the external receiver/reader for observation by a practicing physician or a patient.
One such pressure sensor formed using a MEMS technique has an inductive and capacitive nature. The sensor acts as an inductor (L) and a capacitor (C) connected together in parallel, commonly called an LC tank circuit. The geometry of the sensor allows for the deformation of a capacitive plate with increased pressure. This deformation leads to a deflection of the plate and hence a change in the capacitance value of the system. The LC tank circuit also generates an electronic resonating frequency. This resonating frequency is related to the inductive and capacitance values of the circuit and will change with the deflection of capacitor plates under changing pressure. This emitted resonating frequency signal is received by an external wireless receiver/reader and deciphered into a correlative pressure reading.
Such sensors may also include wireless data transmission ability. The device may require no battery or internal power. Rather, the sensor may be powered by an inductively coupled electromagnetic (EM) field that is directed towards the inductor coil, the receiver/reader device may provide the electromagnetic field by generating a radio frequency (RF) burst or other signal. The inductor receives energy from the EM field to charge the capacitor, where the value of the capacitance varies with environmental pressure. When the EM field is removed, the inductance and capacitance form a parallel resonant circuit to radiate energy through the inductor which acts as an antenna. This oscillating circuit will then produce inherent RF signals, whose frequency is proportional to the capacitive values of the sensor. The inductor coil may serve both as an inductor creating the oscillating RF signals having a frequency proportional to the capacitance of the sensor at a certain pressure, and as an antenna coil emitting the RF signal generated by the LC tank circuitry.
In one embodiment, the pressure sensor may include an inductor/capacitor circuitry assembled in a parallel configuration, or may include a piezoelectric, piezo-resistive or capacitive pressure sensor. In the inductor/capacitor circuitry, the resonating frequency of the energized circuit will change with the internal pressure of the heart. The sensor transmits sensed or detected pressure readings wirelessly to an external system receiver through RF signals without the requirements of an internal powering system. In a particular embodiment, the sensor may be energized through electromagnetic fields that are directed to a circuitry of the sensor.
The pressure sensor may be configured to provide a working surface that is exposed to blood inside the heart or vasculature. This exposure to the internal blood environment exposes the pressure sensor components to the pressure of the blood and allows the pressure sensor to measure and record a corresponding pressure measurement and transmit the pressure measurement to the user.
The cardiac pressure monitoring device may be used as a long-term care monitoring device for patients with chronic heart disease, however the cardiac pressure monitoring device may also be used as a short-term care monitoring device. The pressure data obtained by the sensor/reader system may allow caregivers and clinicians to obtain additional diagnostic data for the patient at a reduced cost compared to other systems and methods.
Although the following disclosure describes a sensor and reader system that is configured to measure and/or monitor an internal fluid pressure within the cardiovascular system to obtain data for guiding therapy, it should be apparent to those skilled in the art that the system as described herein may be configured to measure one or more physical, chemical, and/or physiological parameters or variables to facilitate obtaining data for temperature analysis, blood chemical analysis, blood osmolar analysis, and cellular count analysis, for example. It may also be configured to measure parameters in non-medical applications. The pressure monitoring device may include a pressure sensor, an optical sensor, a biochemical sensor, a protein sensor, a motion sensor (e.g., an accelerometer or a gyroscope), a temperature sensor, a chemical sensor (e.g., a pH sensor), and/or a genetic sensor, for example.
Current designs for passive sensor readers, such as those disclosed in commonly owned U.S. Pat. No. 8,154,389 filed on Apr. 7, 2008, U.S. Pat. No. 8,432,265 filed on Mar. 19, 2012, U.S. Pat. No. 8,493,187 filed on Mar. 19, 2010, and U.S. Pat. No. 8,570,186 filed on Apr. 25, 2012 are incorporated by reference herein. These patents disclose systems configured to communicate wirelessly with a sensor at a remote location and obtain a reading. The reader may be deployed in a use environment and be required to maintain functionality and accuracy over time with few, or no, maintenance activities performed on the reader throughout its rated lifetime. However, there is a need for a simple, inexpensive system and method for testing the reliability of the reader in the field, to ensure functionality and accuracy.